1. Field of the Invention
The invention relates to using cyanodithiocarbimates to control fungal growth on animal hides during a hide tanning process. More particularly, the invention relates to the protection of wet blue hides from fungal attack and degradation.
2. Background of the Invention
Leather, as is well known, is produced from animal hides through a multi-stage process which begins with the tanning of hides. Hides are a complex combination of fats and proteins (elastin, collagen, keratin) which provide for the structure and flexibility of the hide. A leather tanning process involves large shifts in temperature, pH, and types of chemistry employed. Leather tanneries are also unique environments for microbial growth. Leather tanneries are relatively open industrial systems which, as part of the tanning process, have a large influx of microorganisms from the process water, the hides or skins, as well as the other process materials. The incoming organisms vary with the incoming materials. Seasonal climate changes also affect the microorganisms in a tannery's process water.
A typical hide tanning process contains several stages to process the hide into an intermediate form, a tanned hide. In a typical chrome tanning process these stages include, for example, washing the hide with water and surfactant to remove debris; soaking the washed hide in an aqueous enzymatic bath (water, enzyme (protease), surfactant and alkali) to rehydrate the hide and remove interfibrillary proteins; dehairing the hide in a “hairburn” step using amines, sodium sulfide; and lime (CaO); deliming the hide by the addition of ammonium sulfate, water and multiple washes to remove alkali used during hair removal and adjust the pH of the hide; bating the hide (addition of a protease) to remove partially hydrolyzed collagen and keratin; pickling the hide (lowering the pH of the hide to prepare for the chrome tanning) by using an acid such as sulfuric acid; tanning by addition of chrome and basification (chrome fixation) by adding magnesium oxide; followed by rinsing. The resulting hide often has a blue color. At this stage the processed, or chrome-tanned, hide is known as a wet blue hide. The wet blue is only an intermediate which is normally stored or shipped to another location for further processing and finishing into leather.
Other methods of tanning, in addition to chrome tanning (which produces “wet blue” hides), which need protection from fungal contamination, include vegetable tanning, “wet white” tanned hides (including aluminum tanning, zirconium tanning, aldehyde tanning, and phosphonium tanning), and oil tanning. Examples of metal free tanning include use of oil, aldehyde or phosphonium salts. The tanned hides resulting from these tanning processes are known as vegetable-tanned hides, wet white hides and oil-tanned hides.
Each stage of a tanning process is a unique chemical environment as well as a unique microbiological environment. The microbial environment in a tanning process impacts the hide being processed. A hide does not typically need to be protected from fungal attack during the tanning process. After the tanning to yield a tanned hide the hide becomes subject to fungal attack and yields visible mold. It is also during the storage/shipping of tanned hides when fungal infestation occurs. A tanned hide is warm, moist, and a source of nutrients which promote fungal growth and mold which disfigures the appearance of the wet blue hide.
Biocides are used to protect tanned hides from fungi. Fungal attack discolors tanned hides, sometimes causing black spots. Aspergillus niger is a typical fungus that attacks leather products causing discoloration. This fungal discoloration is difficult to remove and can spoil tanned hides or the finished leather.
To protect the tanned hides (such as wet blue hides, vegetable-tanned hides, wet white hides, and oil-tanned hides), a fungicide is added to a tanning liquor used in the tanning process. Fungicides are often added during one of the late stages of the tanning process. Often a fungicide is added during the pickling stage which occurs under very acidic conditions. The fungicide must survive on the surface of the tanned hide to protect the hide from the time the tanning process is complete until the tanned hide is finished into true leather. To do this, a fungicide should itself survive in the chemical environment of the stage in which it is added as well as in the chemical environment of any subsequent stages. In order to be effective then, the fungicide should be stable under acid conditions, stable to UV-light, relatively unreactive with other tanning process chemicals, and have a high affinity for the hide being tanned. Other segments in the tanning process where fungicides could be employed include: prolonged shelf life of leather finish, fat liquor preservation and during the fatliquoring process.
A need exists in the leather industry for compounds that not only are effective fungicides but also survive the chemical environment of the hide tanning processes. This invention answers that need.